This invention relates generally to communication systems, and more particularly to echo cancellation in communication systems.
In order to make high data rate interactive services such as video and internet access available to more residential and small business customers, high-speed data communications paths are required. Although fiber optic cable is the preferred transmission media for such high data rate services, it is not readily available in existing communication networks and the expense of installing fiber optic cabling is prohibitive. Current telephone wiring connections, which consist of copper twisted-pair media, were not originally designed to support the data rates or bandwidth required for interactive services such as video on demand or even high speed internet connections. Asymmetric Digital Subscriber Line (ADSL) technology has been developed to increase the effective bandwidth of existing twisted-pair connections, allowing interactive services to be provided without requiring the installation of fiber optic cable.
Discrete multi-tone (DMT) is a multi-carrier technique that divides the available bandwidth of twisted-pair copper media connections into mini-subchannels or bins. The DMT technique has been adopted in the American National Standards Institute (ANSI) T1.413-1998 standard (ADSL standard). In the ADSL standard, DMT is used to generate up to 250 separate 4.3125 kilohertz subchannels from 26 kilohertz to 1.1 megahertz for downstream transmission to an end user. Likewise, DMT is used to generate 26 subchannels from 26 kilohertz to 138 kilohertz for upstream transmission by an end user. The asymmetric transmission protocol implemented by the ADSL standard generally has a higher rate of data transmission from a central office to a remote terminal and a lower rate of data transmission from a remote terminal to a central office. As a result, different processing sequences are required at the remote terminal and central office ends.
One of the problems that ADSL systems must address is the problem of echo. Echo occurs when the frequency spectra of the upstream and downstream signals overlap. Such overlapping spectra are possible in some implementations of ADSL. However, it is possible to cancel some or most of this echo by using digital signal processing (DSP) techniques. Using this technique the transmit signal which causes the echo is used to estimate the echo component of a received signal. This estimated echo component is then subtracted from the received signal to form an estimate of the true far-end signal.
These systems conventionally have a training period at startup during which the characteristics of the channel are modeled. More sophisticated systems not only perform training at startup, but also adapt the coefficients continually. This adaptive echo cancellation allows for changes in parameters over time. For example, as transmission takes place a transmission line coupler will typically heat up. The result of this heating is that its characteristics change, so that the coefficients developed during the training period may no longer accurately cancel the echo.